1. Field of the Invention
The present invention relates to an image output apparatus.
2. Description of the Prior Art
An image reader detects an original to be read with an image sensor and provides the image data to an output apparatus such as a printer wherein a hard copy on a paper can be obtained.
A copier scans an original to be copied, forms the image of the original on a photoconductor, and transfers the image to a paper as a hard copy.
An original should be placed on the platen in a normal position designated by a reference scale in both image output apparatus. It is a troublesome work to place an original accurately in the normal position. However, if the positioning is not accurate, the reproduced image gets out of position, or it can have certain defects.
A Japanese patent laid open publication No. 173959/1983 discloses an image output apparatus wherein an original can be placed in any place. However, this apparatus needs another operation to designate the area of the image.
When an original is illuminated or is exposed the intensity of the illumination should be stable at a pertinent level in order to get a hard copy at a predetermined density level. However, an exposure lamp deteriorates gradually as time passes. Further, an exposure lamp which is fed with alternating electric current has ripples in the intensity of illumination in correspondence with the frequency of the commercial electric source. Therefore, it is desirable to get a copy with a constant density level regardless of the change due to the above-mentioned factors. Especially in the present invention, which will be explained later, wherein the image area on an original is decided automatically from the change in the density in the original in order to avoid any troubles with the positioning of the original, the density of the original should be detected accurately for the decision of the image area regardless of the change in the intensity of illumination.